Fiber optic cables include optical waveguides such as optical fibers that transmit optical signals, for example, voice, video, and/or data information. One type of fiber optic cable configuration includes an optical waveguide disposed within a tube or cavity, thereby forming an assembly. Generally speaking, the tube protects the optical waveguide; however, the optical waveguide must be further protected within the tube. For instance, the optical waveguide should have some relative movement between the optical waveguide and the tube or cavity to accommodate bending. On the other hand, the optical waveguide should be adequately coupled with the tube or cavity, thereby inhibiting the optical waveguide from being displaced within the tube or cavity when, for example, pulling forces are applied to install the cable. Additionally, the tube assembly or cavity should inhibit the migration of water therein. Moreover, the fiber optic cable should be able to operate over a range of temperatures without undue optical performance degradation.
Conventional optical tube assemblies meet these requirements by filling the tube with a thixotropic material such as grease. Thixotropic materials generally allow for adequate movement between the optical waveguide and the tube, cushioning, and coupling of the optical waveguide. Additionally, thixotropic materials are effective for blocking the migration of water within the tube. However, the thixotropic material must be cleaned from the optical waveguide before connectorization of the same. Cleaning the thixotropic material from the optical waveguide is a messy and time-consuming process. Moreover, the viscosity of thixotropic materials is generally temperature dependent. Due to changing viscosity, the thixotropic materials can drip from an end of the fiber optic cable at relatively high temperatures and the thixotropic materials may cause optical attenuation at relatively low temperatures.
Cable designs have attempted to eliminate thixotropic materials, but the designs are generally inadequate because they do not meet all of the requirements and/or are expensive to manufacture. One example that eliminates the thixotropic material is U.S. Pat. No. 4,909,592, which discloses a tube having conventional water-swellable tapes and/or yarns disposed therein. For instance, conventional water-swellable tapes are typically formed from two thin non-woven layers that sandwich a water-swellable powder therebetween, thereby forming a relatively thin tape that does not fill the space inside a buffer tube. Consequently, conventional water-swellable tapes do not provide adequate coupling for the optical waveguides because of the unfilled space. Additionally, the space allows water within the tube to migrate along the tube, rather than be contained by the conventional water-swellable tape. Thus, this design requires a large number of water-swellable components within the tube for adequately coupling the optical fibers with the tube. Moreover, the use of large numbers of water-swellable components inside a buffer tube is not economical because it increases the manufacturing complexity along with the cost of the cable.
Another example that eliminates the thixotropic material from a fiber optic cable is U.S. Pat. No. 6,278,826, which discloses a foam having a moisture content greater than zero that is loaded with super-absorbent polymers. The moisture content of the foam is described as improving the flame-retardant characteristics of the foam. Likewise, the foam of this design is relatively expensive and increases the cost of the cable.